In association with need diversification, such as conserving resources with electrographic image printers, demand for printing on both sides (double-sided printing) of a recording medium is becoming greater. In addition, maintaining print quality at the time of double-sided printing is also important, as are preventing misalignment of printed positions on both sides and preventing a difference in print size. However, in a medium heated by a heat fixer, moisture contained in the recording medium evaporates due to the heat, and as a result the recording medium shrinks after the fixing of front surface printing. Then, when printing is performed on a rear surface, it causes the printed position to shift and a difference in image size on the two sides.
Further, in many cases, a carrying direction is reversed and the recording medium is inverted by an inversion mechanism for double-sided printing. Therefore, a back end side at the time of printing the front surface will be a front end side at the time of printing the rear surface. Consequently, when the size of the medium is not uniform, a printing start position on the rear surface to be printed next may not be matched with the back end of the front surface printed first, and as a result, print margins are to be shifted on the two sides.
Japanese laid-open application publication number 2005-193615 discloses technology to make positional differences of the same image difficult to be realized in the case of printing both sides of the recording medium. According to the above Japanese laid-open publication, a medium supply detecting unit is arranged in the vicinity of an inlet of an image forming unit in an image forming device, the recording medium length is detected, and shrinkage of the recording medium before and after a fixing process of a recording medium to be carried first is calculated. Then, writing start timing to a recording medium to be carried next is corrected based upon the calculated shrinkage.
However, in the above cited patent reference, because the calculation result of the shrinkage is applied to the recording medium to be carried next, the writing start timing of the recording medium to be carried first is not always corrected.